Interferometry is a non-contact method of measuring surface profile and interferometric profilers have become widely used instruments for analysis and quality control in a range of industries. Several techniques have been developed for calculating surface topography from optical interference data.
Phase-shifting interferometry (PSI), for example, is based on changing the phase difference between two coherent interfering beams at a single wavelength (λ) (or a narrow bandwidth of wavelengths) in a known manner, for example by changing the optical path difference (OPD) either continuously or discretely with time. Several measurements of light intensity with different OPD values, usually equally spaced, at a pixel of a photodetector can be used to determine the phase difference between the interfering beams at the point on a test surface corresponding to that pixel. Based on such measurements at all pixels with coordinates (x,y), a phase map φ(x,y) of the test surface can be obtained, from which very accurate data about the surface profile may be obtained using well known algorithms. Many algorithms have been developed in the art for calculating surface topography from optical interference data (U.S. Pat. No. 2009/0018786-A1). PSI provides a vertical resolution on the order of 1/1000 of a wavelength or better; thus, it is well suited for characterizing smooth, well-reflecting surfaces. At the same time, the PSI technique has a limited vertical range of application because of the so-called 2π ambiguity; i.e. the fact that the phase shift between two beams is repeated with 2π periods every time the OPD exceeds a distance of λ/2. This “phase wrapping” behaviour of PSI leads to ambiguity in the measurements of the surface profile when the surface features are higher than λ/2. Thus, in practice, conventional PSI techniques are generally best-suited to measurements of fairly smooth and continuous surfaces because only in such cases can phase-unwrapping algorithms be applied to reconstruct the surface shape.
Large-step, rough, or steep-surface measurements, on the other hand, have been traditionally carried out with white-light (or broadband-light) vertical-scanning interferometry (VSI). As conventionally implemented, VSI uses a white-light source and the reference arm of the interferometer is scanned vertically with respect to a stationary test sample (or vice versa). In VSI, the degree of contrast of fringes produced on the detector by two interfering beams (instead of their phases) is measured as a function of distance between the reference and test surfaces to obtain information about the test surface. The contrast of a VSI interferogram is maximum when the OPD approaches zero and the test surface topography may be reconstructed by determining the peak position of the envelope of the interferogram for each detector pixel. The VSI approach overcomes the limited scanning range associated with PSI techniques, but suffers from significantly lower resolution (about 3 nm) and, therefore, is not as precise as PSI.
Together, PSI and VSI make it possible to measure most samples, although there may remain difficulties with measurement of samples that combine smooth surfaces with large profile gradients. Measuring the profile discontinuities of such samples requires the large scanning range of VSI, while characterizing a smooth surface texture requires a PSI resolution. This problem has been recently addressed by the development of an enhanced VSI algorithm (referred to as EVSI) that combines both PSI and VSI, as well as high-definition VSI (referred as HDVSI), as disclosed in U.S. Pat. No. 7,605,925, the disclosure of which is incorporated herein by reference.
One of the problems with conventional PSI and VSI techniques is that the sample scanning and data acquisition steps are very time-intensive. For instance, the measurement of a surface of polymeric film having dimensions of 32 cm×32 cm at a sufficiently high resolution (for instance at ×50 magnification) would take very many days, which is prohibitively long for most applications.
It is for this reason that conventional methods of interferometric surface characterisation have scanned only a fraction of the sample surface at isolated and effectively random locations, and then averaged the surface topography over that selected set of small surface areas. The resulting topographical characterisation is therefore extrapolated from selected locations and merely representative of the surface, rather than a true quantification of the surface topography of the sample.
This is a particular problem for polymeric films, as opposed to a machined metallic surface for instance. Surface features in polymeric films result either from irregularities in the polymeric material from which the film is made, or from the processes used in their manufacture, or from extrinsic material deposited on the film surface during film manufacture, processing or storage, or from combinations thereof. The surface features in polymeric films are therefore typically random and irregularly spaced, and so quantification of the surface topography of a commercially relevant area of the film surface has greater value, for instance in research and development of new film applications and in quality control, than an extrapolation from isolated and random locations.
It would be desirable to be able to topographically quantify the entire surface of a large area of sample surface at high resolution and in a speed-efficient manner.
It would also be desirable to be able to topographically characterise the dominant surface features of a sample surface in a precise, accurate and absolute manner at high resolution.